Q15-15-33E - Organic Chemistry (Mcmurry)

Question

Which would you expect to be most stable, Cyclononatetraenyl radical, cation, or anion?

Step-by-Step Solution

Answer

Cyclononatetraenyl cation            Cyclononatetraenyl radical,       Cyclononatetraenyl 

        (Most stable)                                                                                          anion

Aromaticity

The aromaticity of a molecule is decided by Huckel’s rule$\left(4n+2\right)\mathbf{\pi }$. According to this rule, an aromatic compound is a planar, conjugated system that contains a total of $\left(4n+2\right)\mathbf{\pi }$delocalized electrons (where n is an integer). Molecules with $\mathbf{4}\mathit{n}\mathbf{\pi }$

electrons are nonaromatic systems. These systems are called antiaromatic and are often very unstable.

Cyclononatetraenyl radical

Cyclononatetraenyl cation has 8 pi electrons or $4n\mathrm{\pi }$ for n=2, thus making it antiaromatic and highly unstable. On the other hand, cyclononatetraenyl anion has 10 pi electrons or  $\left(4n+2\right)\pi$electrons for n=2, which makes it aromatic, and highly stable, followed by cyclononatetraenyl radical, which is neither aromatic nor antiaromatic.

Therefore, Cyclononatetraenyl cation is the most stable among cyclononatetraenyl radical, cyclononatetraenyl cation, and cyclononatetraenyl anion. The structures are as shown:

Cyclononatetraenyl cation            Cyclononatetraenyl radical,       Cyclononatetraenyl 

        (Most stable)                                                                                        anion